This invention relates generally to filtration technology and, more specifically, to a method and apparatus for increasing the efficiency of a filtration system employing multiple stages.
While the invention will be described with particular reference to spiral wound filtration membranes and modules containing spiral wound membranes, it is to be understood that the invention is applicable to other types of filtration where multiple modules are employed.
A typical spiral wound filtration module is shown and described in U.S. Pat. No. 4,301,013, issued Nov. 17, 1981. Spiral wound filtration membranes have many advantages as a result of their high surface to volume ratio, low holdup volume, and generally uniform flow patterns. Spiral wound membranes are particularly useful in ultra filtration processes such as those used in the dairy industry for concentrating cheese whey as well as the separation and concentration of other separable components of a fluid milk product.
A typical spiral wound filtration system employs a spiral wound membrane that is wound around a perforated central tube and enclosed within a housing. Typically, multiple spiral wound membranes, also known as modules, are coupled in series within a single housing and one or more housings may also be joined together for carrying out the filtration process.
Two well-known configurations for filtration modules that are to be employed in a multiple stage filtration system are the so-called "once through" concept or "Christmas tree" design and the "recirculating" design.
In the well-known "once through" design, a plurality of housings each having multiple modules disposed therein are placed in parallel relationship and a feed stream is directed by parallel flow through the housings. The concentrate from the housings is then fed to a second stage which normally has fewer housings than the preceding stage. The feed is again directed by parallel flow through the second stage housings and the concentrate from this stage is directed to third stage. The third stage has fewer housings than the second stage and the number of housings is continually reduced in each successive stage until the final concentration is reached. The concentrate from the last two housings is the end product. This particular design is theoretically the most efficient means of operating a multiple stage filtration system, but it is limited in its versatility to process alternative feed rates. Also, as the permeability of the filtering media decreases during use, it is necessary to increase the system pressure to compensate. As the pressure increases, the flow rate may decrease because of limitations of the equipment thereby creating a counterproductive situation. Thus, the objective of maintaining a predetermined cross-flow velocity through the system is only partially met through compromising one factor in favor of another.
Another well-known design for a filtration system employing multiple modules is referred to as the "recirculating" design. A typical arrangement for a recirculating filtration system is shown in FIG. 1 of the drawing and will be discussed in greater detail hereinafter.
Briefly stated, the recirculating in series design employs a recirculating pump in association with each stage and a portion of the concentrate from each stage is recirulationd to the feed stream of that same stage. The recirculating design offers greater flexibility in controlling pressure and flow rate, but is also more expensive because of the additional pumping energy that is required.